Vessels made of plastic (for example, bottles and canisters) afford the following advantages with respect to known vessels made of glass or sheet metal: a low weight, a high degree of resistance to breakage and a reduced tendency to corrode combined with an advantageous performance when the vessel bursts. However, plastic vessels are disadvantageous because of the unsatisfactory barrier diffusion effectiveness of the plastic materials used. Depending upon the material used, the plastics permit more or less large quantities of oxygen, carbon dioxide, water vapor, solvents (alcohol, aromatic substances) and the like to diffuse.
Accordingly, experiments were made in plastic vessels to improve the barrier effectiveness against diffusion. Plastic vessels, which comprise one or more multi-layered laminates made of different plastics have not been accepted because the raw materials of the vessels are, in part, expensive and the vessels are complicated and therefore expensive to manufacture. Furthermore, the diffusion is still too great even for these laminate vessels so that a true long-term stability of, for example, more than a year is not obtainable.
Another solution comprises applying a so-called barrier layer to the surface of a plastic vessel. This barrier layer is intended to prevent the diffusion of gases and, if required, to prevent an attack by the material stored in the vessel on the plastic. In U.S. Pat. No. 4,552,791, a vessel is described having an outer surface which has a barrier coating of inorganic oxides. The function of the barrier coating is so described that the inorganic oxides partially penetrate into the polymer and clog the interstices and the porosity between the polymer chains. Most inorganic or metal oxides are suitable for this purpose. Oxides of silicon, aluminum, titanium and tantalum are preferred. The thickness of the oxide layer extends from less than 50 nm to over 500 nm. The coating is applied by means of a PVD (physical-vapor deposition) method by vaporization of the layer material in an electric field with the layer material being ionized by means of gas discharge. Although the diffusion barrier effectiveness is improved by a factor of 3 by this barrier coating, the result nonetheless needs improvement. For small layer thicknesses, the barrier effectiveness against diffusion is not satisfactory and for large layer thicknesses, the problem is presented that the layer peels especially when the vessel is subjected to sterilization in an autoclave.
European patent publication 0,460,796 (corresponding to U.S. patent application Ser. No. 07/513,353, filed Apr. 20, 1990, now U.S. Pat. No. 5,177,739 and Ser. No. 07/513,302, filed Apr. 20, 1990, now U.S. Pat. No. 5,084,356) discloses a vessel having an improved barrier effectiveness. This vessel has a diffusion barrier coating which comprises a packet of layers of two sequentially arranged inorganic barrier layers of different composition, namely, SiO and SiO.sub.2. The SiO and SiO.sub.2 layers alternate. The SiO layers have a thickness of 10 to 75 nm and the SiO.sub.2 layers have a thickness of at least 20 nm. A considerable improvement of the barrier effectiveness against diffusion is provided by the buildup of two layers of different composition. However, the disadvantage is presented that a relatively thick coating is required for providing a good barrier effect. A thick coating tends, because of its brittleness, to form fissures and to peel off because the bendability and the capability of withstanding inner stresses is reduced with increasing thickness.